Merge branch 'refs/heads/t/simple_error_codes'

Two patches which get rid of the concept of per-subsystem error
codes. The host-compiled error2.c program can be removed, configure.ac
and error.h simplified.

The merge conflicted because both sides modified error.h, but this
was easy to resolve.

* refs/heads/t/simple_error_codes:
  Sort errors alphabetically.
  Simplify the error subsystem, get rid of error2.[ch].